Fix bug in paas-clean
Fix bug in paas-clean

Allow the `cf logout` command to fail by using Make specific syntax
[[1]]. If for some reason we aren't already logged in to PaaS, cf logout
will complain that it can't log out.

[1]: https://www.gnu.org/software/make/manual/html_node/Errors.html#Errors

alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws